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Russia is "responsible" for a nerve agent on a former KGB agent and his daughter in the UK, which has caused a diplomatic crisis between London and Moscow.

"The United States believes that Russia is responsible for the attack on two people in the United Kingdom, using a military-grade nerve agent," US Ambassador Nikki Haley told an emergency council meeting.

She urged the UN Security Council to hold Russia accountable for the use of the nerve agent Novichok in the town of Salisbury, which left former spy Sergei Skripal and his daughter in a critical condition.

The UK has blamed Moscow for the attack and expelled 23 Russian diplomats over the 4 March poisoning.

US President Donald Trump has not directly connected the attack to Russia, while his former Secretary of State Rex Tillerson was fired shortly after blaming Moscow.

Now the US appears to be taking a tougher line against Russia, saying Moscow must be held to "account for its actions".

"If we don't take immediate, concrete measures to address this now, Salisbury will not be the last place we see chemical weapons use." 

Russia has denied it was behind the attack and asked the UK to send samples of the chemical. 

London has ignored the request and announced a range of sanctions against Russia after Moscow did not respond to its demand for answers on the issue.

UK Deputy Ambassador Jonathan Allen said the attack was "state-sponsored".

"We ask you today to stand by us," Allen told the council.

"This was a reckless and indiscriminate act that put at risk the lives of civilians."

Russia continued to deny their role in the killing.

"Russia had nothing to do with this incident," said Ambassador Vassily Nebenzia. "We have nothing to fear, nothing to hide."